Fair value measurements (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance of other assets and other liabilities	$ 20,435	$ 16,147	$ 6,923	$ 2,244
Total gains (losses) realized/ unrealized:
Included In Earnings of other assets and other liabilities	45,486	22,800	(72,517)	43,244
Transfer in and/or out of Level 3 of other assets and other liabilities	(31,870)	(25,776)	(45,389)	(32,317)
Ending Balance of other assets and other liabilities	34,051	13,171	34,051	13,171
Changes in unrealized gains (losses) included in earnings related to other assets and other liabilities still held at end of period	28,904	10,252	33,647	13,139
Privately issued residential [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,110,695	1,391,878	1,136,256	1,435,561
Total gains (losses) realized/ unrealized:
Included in earnings	(14,627)	(24,530)	(22,304)	(32,071)
Included in other comprehensive income	22,613	38,471	47,683	99,556
Settlements	(50,289)	(99,617)	(93,243)	(196,844)
Transfer in and/or out of Level 3
Ending Balance	1,068,392	1,306,202	1,068,392	1,306,202
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period	(14,627)	(24,530)	(22,304)	(32,071)
Privately issued commercial [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	13,435	20,467	15,029	22,407
Total gains (losses) realized/ unrealized:
Included in other comprehensive income	763	(1,400)	1,111	(1,482)
Settlements	(2,071)	(1,834)	(4,013)	(3,692)
Transfer in and/or out of Level 3
Ending Balance	12,127	17,233	12,127	17,233
Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	58,184	114,265	52,500	110,756
Total gains (losses) realized/ unrealized:
Included in other comprehensive income	(2,712)	3,372	3,711	7,206
Purchases		50,790		50,790
Sales		(105,643)		(105,643)
Settlements	(374)	(1,183)	(1,113)	(1,508)
Transfer in and/or out of Level 3
Ending Balance	$ 55,098	$ 61,601	$ 55,098	$ 61,601